1. Field of the invention:
Membrane gas separation technology holds promise as inexpensive and energy-saving technology for separatng gasses and active studies are being made to realize industrially feasible processes. Primary to attaining this object is the development of a membrane material having high capabilities of both gas permeation and separation, and the development of a gas separation module that is compact and simple in structure and which is yet inexpensive and highly reliable. The present invention relates to the achievement of the latter requirement.
2. Description of the Prior Art:
Porous membranes are relatively poor in their ability to separate hydrogen from gas mixtures, so the predominant portion of the current R&D efforts on membrane gas separation technology is expended in nonporous polymer membranes. However, membranes of porous inorganic materials used in the separation and concentrating of hydrogen have two advantages not possessed by nonporous polymer membranes, i.e., high heat resistance and high gas permeability. Known porous inorganic membrane materia1s include porous glass, ceramic sinters and metal sinters, and porous glass is the best material since micro pores having diameters ranging from several tens to hundreds of angstroms can bc obtained fairly easily.
A conventional gas separating module using a porous glass membrane is disclosed in Unexamined Published Japanese Patent Application No. 119420/1980. This reference proposes a method of separating and concentrating hydrogen with a module comprising a bundle of porous hollow glass fibers (outside diameter .ltoreq.2 mm, wall thickness.ltoreq.0.8 mm, pore diameter=20-200 .ANG.) encased within a sheath. In accordance with this method, the flow rate of the hydrogen gas that permeates through the fibers per unit volume is increased by reducing the outside diameter and wall thickness while increasing the number of fibers packed in the sheath. In the Example, there is shown a module consisting of as many as about 2.times.10.sup.5 porous glass capillaries (OD=0.3 mm, wall thickness=0.05 mm, average pore dia.=43 .ANG.) incorporated within a metal pipe.
A schematic of this module is shown in FIG. 10; a metal pipe 23 has incorporated therein a fluid distributing pipe 24 and ca. 2.times.10.sup.5 porous glass capillaries 25. The capillaries are supported by a sheet 26 at several points along their length so that the individual capillaries are spaced apart by a small distance. The top of the capillaries penetrates through a support 27 and is open to a permeate gas receptacle 28, whereas the bottom of the capillaries is securely embedded in a support 29. The top of the distribution pipe 24 is closed and embedded in the support 27. A feed gas mixture is forced through the distribution pipe 24 and all of the gas is blown against the outer surfaces of the capillaries 25 in the direction indicated by arrow B. In order to ensure that the stream of the gas feed will flow uniformly along the outer surfaces of the capillaries, the distribution pipe must be properly designed and the glass capillaries must also be properly arranged. The hydrogen gas in the feed permeates through the glass capillaries and its concentrate flows in the direction indicated by arrow C and is recovered from the top. The remainder of the feed that does not permeate through the capillaries passes between each capillary and is collectcd as indicated by arrow D after passing through a space 30.
The module structure shown above has various problems.
The first problem is that a compact structure cannot be realized because the glass capillaries must be packed in the metal pipe in an arrangement other than that of "close packing" in order to provide a sufficient space between each capillary for ensuring a gas passage from the outside to the inside of the capillary.
The second problem is associated with the complexity of the work necessary for securing a multiple of porous glass capillaries with a seal. Utmost care and prolonged timc are required for securely attaching the fluid distribution pipe and the great number of glass capillaries to the supports 27 and 29 while arranging them in the proper pattern and without causing damage to any of the capillaries.
The third problem associated with the module is its low durability and heremeticity. One reason for this problem is the high possibility of the porous glass capillaries breaking at the portions embedded within the end seals. Because of their porous nature, the porous glass capillaries are weaker than non-porous glass capillaries. When the module is subjected to temperature cycles, cracks will easily develop in the portions embedded in the end seals because of repeated stress resulting from thermal expansion mismatches between the glass and its supporter, e.g., the sealing material and the metal pipe. This greatly reduces the durability of the module. The other reason for the third problem is that the hermeticity of the module ends is easily impaired. In order to realize efficient gas separation, the space of the outside and the space of the inside of a glass capillary, one side with a higher pressure and the other side with a lower pressure, must be hermetically isolated from each other by the end seals for the glass capillary. However, if several hundreds of glass capillaries are spaced within a metal pipe with the clearance between each capillary held to a minimum in order to obtain a near "close packed" arrangement, air bubbles may be entrapped in the capillary spacing or incomplete sealing of that space may occur, resulting in low hermeticity at the ends of the module.